Micro-inverters or single-panel inverters?

SolarVision consultants take a closer look at inverters and microinverters, and the challenges vs. benefits in integration, cost, and reliability.

by Andy Skumanich, Igor J. Malik, Elmira Ryabova, SolarVision Consulting, Los Gatos, CA USA

This article is a companion piece to the authors’ discussion about PV “balance of system” (BOS), in the January-February issue of Photovoltaics World.)

February 3, 2010 – The most critical elements in the balance-of-system (BOS) hardware, inverters convert DC power into AC power and may provide data monitoring. Inverters are grouped in two major categories — with or without batteries. Grid-connected inverters run at ~96%-97% efficiency, but this drops to 77% for battery-connected (due to the aging effects of the batteries).

Typically, inverters need to be replaced as early as 10 years into a given operating cycle. Inverter designs are optimized for PV installations of different sizes because the system efficiency is pegged to the inverters. Figure 1 shows three different inverter designs, ranging in maximum power load from ~100W for the mini-inverter, to several MW for the utility-scale model.

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An example of the range in inverters (l-r): single panel, home rooftop, and utility (images not to scale).

A point of focus for inverter development has to do with what are termed micro-inverters (Figure 2). For a typical installation, the panels are linked into strings and then connected with parallel strings into the primary inverter. But there are numerous issues with this arrangement. In particular, if there is a shadow, or a fault with a given PV panel, it can take an entire string off line. In addition, if each panel could separately generate AC current, there would be reduced electrical hazard from the panel with several hundred DC volts potential. As a result, there has been an extensive amount of activity in trying to develop inverters for each panel.

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A micro-inverter proto-type from ISET showing an advanced layout (fewer components) and a high-frequency transformer (lower weight). (Courtesy ISET, Michael Viotto)

In this case, the limitations of strings would be eliminated; however, the challenges remain cost benefits, and equally important, reliability. The micro-inverters need to have exceedingly high reliability to match the >20-year panel lifetimes. Otherwise, they have to be replaced and maintained. The overall system costs would be higher if each panel had such a high-reliability micro-inverter, and the benefits of better energy harvesting need to be balanced against the higher equipment and (potential) maintenance costs.

Still, this is a promising avenue of pursuit, and it is likely that ultimately some type of single-panel inverters will become available.

Andy Skumanich received his PhD in physics at the U. of C. Berkeley, and is founder and CEO at SolarVision Consulting, 412 LG Almaden Road, Los Gatos, CA 95032 USA; e-mail askumanich@solarvisionco.com.

Igor J. Malik received his PhD in chemistry at the U. of Illinois at Chicago, and is a principal consultant at SolarVision Consulting.

Elmira Ryabova received her PhD in Physics at Ioffe Physical Technical Institute, and is Sr. Technologist at SolarVision Consulting.

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